[GIS] How to group by attributes and give unique categories in QGIS

fields-attributesgroupingqgis

I have features and each of them has an attribute about what it is. In my case, I have points representing trees and each tree has its species name as an attribute.

I would like to group them by species name and using numbers from 1 to n as seen on picture and if possible, not by hand because sometimes I have hundreds of species. I was thinking of some kind of expression which includes $rownum and tried a few things but no results. SQL queries also came to mind, but I have limited experience with them and I've never given them a try in QGIS.

I really don't want to export data to a spreadsheet, I would like to solve this in QGIS if possible.

Best Answer

There is an easy way that needs two steps from the db-manager and two tables that can be deleted in the end(there is a way with virtual tables, but i don´t get them to work properly from the db-manager).

go to db-manager and then to virtual layers and select your table with the trees; then start the sql-window
Enter into the query field: SELECT distinct(species) FROM "your table name"

run it, and save it as a new table (for example temp1) with the button down below

now open the same way the query again for your temp1-table and enter:

SELECT species,

rowid AS number from temp1

run it and save it as temp2

now you can join your table temp2 to your species-table by using the column species (via the joins function from your layer-preferences); Qgis does create a 1:n join and therefore the numbers are copied for each species
use the field calculator to add the numbers to your species table, then you can delete the temp1 and temp2

Related Solutions

[GIS] How to build a query/expression to select specific points within a buffer? (in QGIS)

This can be done using the Processing Graphical Modeler to set up a custom chain. Here's a pic:

From left to right we have three inputs - the tree layer (a Vector Layer Input Object), the buffer size (a Numeric Input) and the tree height (a String Input). The tree layer is then selected by its height attribute. The selected trees are then buffered by the value of the buffer_width input - this is saved as a layer treebuff (remove that output if you don't need it, I just used it for checking). Then the Select By Location process sets the selection to those trees inside the buffer region (set the first input to the trees, the second to the buffer, and the predicate to "contains")

Here's the resulting output:

Large trees are drawn large, and you can see the buffer zone and the selection now includes the large trees and their neighbours within the buffer.

Create (autogenerate) report in QGIS, with a map for each type of feature

You could use a virtual layer (i.e. vtrees) to group your species into multipoint geometries.

SELECT species, CastToMultiPoint(Gunion(geometry)) as geom FROM trees GROUP BY species

Style your layer according the species field and the variable @atlas_pagename.

if("species" = @atlas_pagename, "species", "")

And create your atlas with layer vtree as coverage layer and species as page name.

Best Answer

Related Solutions

[GIS] How to build a query/expression to select specific points within a buffer? (in QGIS)

Create (autogenerate) report in QGIS, with a map for each type of feature

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